Open Shortest Path First (OSPF) is a link-state based interior gateway protocol developed by the Internet Engineering Task Force (IETF) organization. At present, OSPF Version 2 (RFC 2328) is used for IPv4 protocol. OSPFv3 is developed on the basis of OSPF, in order to support IPv6.
OSPFv3 protocol divides an autonomous system into different areas. The term “area” refers to that routers are divided logically into different groups, with each group being identified by an area ID. For example, a network system is divided into five areas Area 0 through Area 4, wherein one of the areas (such as Area 0) is connected to all the other areas Area 1 through Area 4 and is called the backbone area. The link state information of OSPFv3 diffuses only within the range of an area. Each area has an independent link state information library, and an independent Shortest Path First (SPF) calculation is performed within each area.
Link State Advertisement (LSA) is the main source for OSPFv3 protocol calculating and maintaining the routing information. RFC5340 defines several types of LSAs including Router-LSA , Network-LSA, Inter-Area-Prefix-LSA, Inter-Area-Router-LSA, AS-External-LSA, Link-LSA and Intra-Area-Prefix-LSA.
Intra-Area-Prefix-LSA: OSPFv3 moves the IP prefix directly reachable to the present machine which was notified by the stub link of the Router-LSA and the IP network segment of the broadcast network which was notified by the Network-LSA in OSPF to this type of LSA. This LSA propagates within an area, and is used for notifying in an area the IP prefix that is directly reachable within the area. Because this LSA can be generated according to a stub link, or generated according to a network LSA, so this LSA further functions as a reference type: being 2001 in case reference is made to a router LSA, and being 2002 in case reference is made to a network LSA.